Remote absorption spectroscopy by coded transmission

Abstract

Remote absorption spectroscopy uses coded electromagnetic transmission directed through a medium under investigation to one or more remote receivers. The coded transmission includes at least one wavelength coincident with an absorption band of interest and one wavelength in an off-line band and a predefined relationship between spectral components in and outside the absorption band is controlled. The relationship between spectral components may be evaluated at the receiver to determine whether deviation thereof from the controlled relationship at the transmitter exists at the receiver. The deviation of the received optical signal from the prescribed relationship is processed to indicate the absorption of the radiation in the absorption band.

What is claimed is: 
     
       1. An apparatus comprising:
 a generator configured to generate multispectral electromagnetic radiation having at least one spectral component having a wavelength coinciding with an absorption band of a known substance and at least one spectral component having a wavelength coinciding with an off-line band that is removed from the absorption band wavelength; 
 a transmitter configured to:
 modulate the generated spectral components with respective modulation codes that define a predetermined relationship between spectral content of the modulated spectral component having the absorption band wavelength and the spectral content of the modulated spectral component having the off-line band wavelength; 
 modify the modulation codes in response to determining that the relationship between the spectral content of the modulated spectral component having the absorption band wavelength and the spectral content of the modulated spectral component having the off-line band wavelength is other than the predetermined relationship; and 
 transmit the modulated radiation through a medium; and 
 
 at least one receiver spatially isolated from the transmitter and arbitrarily locatable with respect thereto, the receiver configured to:
 accept the transmitted radiation through line-of-sight propagation through the medium; 
 measure the relationship between the absorption band spectral content and the off-line band spectral content of the received radiation; and 
 determine absorption at the wavelength coinciding with the absorption band from a deviation of the measured relationship between the absorption band spectral content and the off-line band spectral content from the predetermined relationship. 
 
 
     
     
       2. The apparatus of  claim 1 , wherein the transmitter includes:
 a radiation channel for each spectral component in the absorption band and for each spectral component in the off-line band, each radiation channel modulating the corresponding spectral component such that the relationship therebetween is maintained. 
 
     
     
       3. The apparatus of  claim 2 , wherein the transmitter includes:
 a modulator in each radiation channel configured to modulate the corresponding spectral component by the respective modulation codes in accordance with respective control signals provided thereto; 
 a detector configured to generate an electrical signal from the modulated radiation incident thereon from the radiation channel of at least one spectral component in the absorption band and the radiation channel of at least one spectral component in the off-line band; and 
 a controller configured to generate the control signals responsive to the detector signal so as to maintain the relationship between the absorption band spectral content and the off-line band spectral content. 
 
     
     
       4. The apparatus of  claim 3 , wherein the controller includes an encoder to generate modulation codes for respective radiation channels and to provide an indication of the codes in the control signals, the encoder modifying a parameter of the codes so as to maintain the relationship between the absorption band spectral content and the off-line band spectral content at the predetermined relationship. 
     
     
       5. The apparatus of  claim 4 , wherein the codes identify the respective radiation channels of the transmitter at the receiver. 
     
     
       6. The apparatus of  claim 4 , wherein the codes are amplitude modulation codes and the parameter thereof is signal amplitude. 
     
     
       7. The apparatus of  claim 4 , wherein the controller includes a discriminator to generate discriminator signals responsive to the signal from the detector such that the discriminator signals represent transmitted energy in the modulated spectral component having the absorption band wavelength and in the modulated spectral component having the off-line band wavelength. 
     
     
       8. The apparatus of  claim 7 , wherein discriminator includes a correlator for each radiation channel, the correlator being tuned to the code assigned to the corresponding radiation channel to provide a corresponding one of the discriminator signals responsive to the detector signal. 
     
     
       9. The apparatus of  claim 3 , wherein the receiver includes:
 a detector configured to generate a receiver signal from the received radiation incident thereon; and 
 an analyzer configured to measure the relationship between the absorption band spectral content and the off-line spectral content from the receiver signal and to compute the absorption at the wavelength coinciding with the absorption band from the deviation of the measured relationship between the absorption band spectral content and the off-line band spectral content from the predetermined relationship. 
 
     
     
       10. The apparatus of  claim 9 , wherein the analyzer includes a discriminator to generate discriminator signals responsive to the receiver signal from the detector, each of the discriminator signals representing energy in a corresponding spectral component of the received radiation including the absorption band spectral component and the off-line band spectral component. 
     
     
       11. The apparatus of  claim 10 , wherein discriminator includes a correlator for each spectral component to be analyzed, the correlator being tuned to the code assigned to the corresponding spectral component to provide a corresponding one of the discriminator signals responsive to the receiver signal. 
     
     
       12. The apparatus of  claim 1 , wherein the transmitter and receiver are independently locatable in a substantially boundless medium. 
     
     
       13. The apparatus of  claim 12 , wherein the at least one receiver comprises a plurality of receivers substantially identical to the at least one receiver. 
     
     
       14. The apparatus of  claim 13 , wherein the transmitter includes a beamformer by which the transmitted radiation is distributed to be simultaneously accepted by the receivers. 
     
     
       15. The apparatus of  claim 13 , wherein the transmitter includes a movable platform by which the transmitter traverses a trajectory through which the radiation transmitted therefrom is accepted by the receivers in a sequence defined by the trajectory. 
     
     
       16. A method comprising:
 generating by a transmitter multispectral electromagnetic radiation including at least one spectral component having a wavelength coinciding with an absorption band of a known substance and at least one spectral component having a wavelength coinciding with an off-line band that is removed from the absorption band wavelength; 
 modulating the generated spectral components with respective modulation codes that define a predetermined relationship between spectral content of the modulated spectral component having the absorption band wavelength and the spectral content of the modulated spectral component having the off-line band wavelength; 
 modifying the modulation codes in response to determining that the relationship between the spectral content of the modulated spectral component having the absorption band wavelength and the spectral content of the modulated spectral component having the off-line band wavelength is other than the predetermined relationship; 
 transmitting the modulated radiation through a medium; 
 accepting at a receiver the transmitted radiation through line-of-sight propagation through the medium; 
 measuring the absorption band spectral content and the off-line band spectral content; and 
 determining absorption at the wavelength coinciding with the absorption band from a deviation of the relationship between the measured absorption band spectral content and the measured off-line spectral content from the predetermined relationship. 
 
     
     
       17. The method of  claim 16 , wherein modulating the spectral components includes:
 encoding the spectral component having the absorption band wavelength and the spectral component having the off-line band wavelength with respective modulation codes; and 
 modifying a parameter of the codes so as to maintain the predetermined relationship between the spectral content of the modulated spectral component having the absorption band wavelength and the spectral content of the modulated spectral component having the off-line band wavelength. 
 
     
     
       18. The method of  claim 17 , wherein:
 measuring the absorption band spectral content and the off-line band spectral content includes correlating a representation of the received radiation with the codes to produce a representation of the received spectral component having the absorption band wavelength and a representation of the spectral component having the off-line band wavelength; and 
 determining the absorption includes determining a difference between the predetermined relationship and the relationship between the spectral component representations produced by the correlating and establishing the difference as the deviation. 
 
     
     
       19. The method of  claim 18  further comprising:
 establishing as the predetermined relationship a predetermined proportionality between energy in the modulated spectral component having the absorption band wavelength and the energy in the modulated spectral component having the off-line band wavelength. 
 
     
     
       20. The method of  claim 19 , wherein the determining of the absorption includes:
 attributing to the absorption by the known substance in the medium the difference between the proportionality of the energy in the received spectral component having the absorption band wavelength to the energy in the received spectral component having the off-line band wavelength and the predetermined proportionality. 
 
     
     
       21. A non-transitory computer readable medium encoded with software comprising processor-executable instructions that, when executed by a processor, cause the processor to perform functions of:
 encoding components of multispectral electromagnetic radiation with a plurality of modulation codes to define a predetermined relationship between spectral content of at least one of the modulated spectral components having a wavelength coinciding with an absorption band of a known substance and the spectral content of at least one spectral component having a wavelength coinciding with an off-line band that is removed from the absorption band wavelength; 
 modifying the modulation codes in response to determining that the relationship between the spectral content of the modulated spectral component having the absorption band wavelength and the spectral content of the modulated spectral component having the off-line band wavelength is other than the predetermined relationship; 
 measuring the absorption band spectral content and the off-line band spectral content of the encoded radiation subsequent to line-of-sight propagation thereof through a medium; and 
 determining absorption at the wavelength coinciding with the absorption band from a deviation of the relationship between the measured absorption band spectral content and the measured off-line spectral content from the predetermined relationship.